The present invention relates to a radio frequency amplifying apparatus to be utilized for a radio frequency heating system for heating and current driving plasma through a supply of radio frequency power to the plasma such as in a tokamak type nuclear fusion system or for an accelerator for accelerating charged particles to high energy particles.
FIG. 10 is a schematic view showing a tokamak type fusion system as a system for confinement of plasma by utilizing a magnetic field, and in such tokamak type fusion system, plasma 101 is confined in shape of torus by a vertical field Bv, a toroidal field Bt in a circumferential direction and a poloidal field Bp created by a plasma current Ip passing inside the torus.
The vertical field Bv is generated, as shown in FIG. 11, by poloidal coils 102 arranged along the torus plasma 101 and the toroidal field Bt is generated by toroidal coils 103 arranged to form a circular solenoid coil means. The plasma current Ip is generated by electromagnetic induction. The plasma current Ip acts to rapidly change current of induction coils 104 arranged at the central portion of the tokamak type fusion system and to induce an electric field in the plasma 101 by time change of the magnetic flux, thereby the plasma current Ip being generated by accelerating electrons by the electric field.
The plasma 101 thus generated in the tokamak type fusion system has temperature merely of about several tens of million degrees as it is, and in order to obtain temperature of about several hundreds of million degrees required for a nuclear fusion reactor, for example, it is necessary to heat the plasma by suitable means. One means for heating the plasma is represented by a so-called ion cyclotron heating method in which a high frequency wave having a frequency synchronized with a turning motion, i.e. cyclotron motion, of ions in the magnetic field is given to cause the frequency resonance and then to apply the high frequency energy to the ions.
FIG. 12 shows a general arrangement of a standard ion cyclotron heating device of the type in which radio frequency power is supplied to four antennas 115. In the ion cyclotron heating device, a small power frequency generated from a source oscillator or generator 110 is divided into four parts and power amplification is carried out by providing phase difference required for the heating of the respective lines by means of phase converters 111. Since the radio frequency power from the source oscillator 110 is very small and it is hence impossible to amplify the power by utilizing a vacuum tube, the power is preliminarily amplified by a pre-amplifier 112 to a suitable level of power and the amplified power is thereafter inputted into a power amplifier 120.
The heating system includes four power amplifiers 120 each being composed of a low power amplifier (LPA) 121, an intermediate power amplifier (IPA) 122 and a high power amplifier (HPA) 123, which amplifies the radio frequency power from 1KW order to MW order.
The radio frequency from the power amplifier 120 is transmitted to the antenna 115 through an impedance matching system (IMS) 114 by way of a transfer line 113. Generally, as an input impedance for the antenna 115 differs from a characteristic impedance, the impedance matching system 114 composed of a stub is arranged on the way of the transfer line 113.
A conventional radio frequency heating system aims mainly to heat the plasma, but recently, there has been studied the radio frequency current driving technology for generating current in the plasma because it becomes possible to pass the current continuously in the plasma of the tokamak type fusion system by utilizing the radio frequency, different from a case of the electromagnetic induction technology. There has been also carried out experiments of the current driving for accelerating electrons by means of the radio frequency in the ion cyclotron heating system. In such case, it is necessary to change the frequency level to that of the heating to avoid absorption of the radio frequency by the ions.
Moreover, in the conventional radio frequency heating apparatus, the electrons are accelerated in one direction. Accordingly, as shown in FIG. 13, when the radio frequency is applied to the plasma from the antenna 115 (115a to 115d, for example), it is necessary to set the phase defference, which is different from that at the time of heating, between the respective antennas 115a to 115d so that travelling waves are formed by the electromagnetic waves. As shown in FIG. 13, the antennas 115a to 115d are connected to the inner conductors 117 and the outer conductors 118 respectively through a return conductor 116.
In the assumption of a future nuclear fusion reactor, it is disadvantageous to separately locate a heating means and a current driving means to the system in view of the reduction of the construction cost and an occupied space, and it is hence significantly required to provide one system which can carry out the heating and current driving operation. In this requirement, it is required for the radio frequency amplifying apparatus to carry out the frequency changing easily with short time.
However, if such system carrying out the heating and current driving operations is to be realized, it will be required to change the frequency and the phase difference during the operation. The phase difference changing is performed by an electronic circuit with relatively short time, but the frequency changing requires relatively much time for the reason described hereunder. Although the radio frequency power has to be stopped during the operation change, the current driving efficiency is extremely lowered by the stopping of the radio frequency power because the temperature of the plasma, particularly the electron temperature, is lowered. For this reason, it is required to make as short as possible the transfer time from the heating mode to the current driving mode.
In spite of the above fact, in order to change the frequency, it is necessary to change not only the oscillation frequency of the source oscillator 110 but also tuning frequencies of output circuits of the respective amplifiers 121, 122 and 123 constructing the power amplifying apparatus 120 as well as the output circuits themselves. For example, with reference to FIG. 14 showing one example of a resonator of the output circuit of the amplifier, there is shown a standard reentrant type cavity resonator 130 in which an inner conductor 131 and an outer conductor 132 are arranged, and when such cavity resonator 130 is utilized, the tuning frequency is changed by changing a distance L by moving a short-circuit plate 133 along the axial direction of the inner and outer conductors 131 and 132.
Accordingly, on the changing of the frequency, a time for moving the short-circuit plate 133 and adjusting the tuning frequency is additionally required. As shown in FIG. 12, in the case of three-stage amplifiers, the tuning must be carried out generally at six portions on the input and output sides, thus requiring much time for the adjustment and hence being inconvenient in operation.
A metallic contact 134 is secured to the short-circuit plate 133 with a constant pressing pressure for ensuring an electric contact, and in general, since a plurality of such metallic contacts 134 are attached along the circumferential direction of the short-circuit plate 133, it is difficult to instantaneously move the short-circuit plate 133 to change the tuning frequency. On the contrary, to make fast the moving speed of the short-circuit plate 133 will damage the contacting performance of the metallic contacts 134, resulting in the lowering of the reliability and shortening of the life time, and in a certain case, some inconvenience may be caused to a driving force generation means for moving the short-circuit plate 133.